Note: Descriptions are shown in the official language in which they were submitted.
1~47580
This invention relates to nonmagnetic steel having
a low thermal expansion coefficient and a high yielding point
and a method of manufacturing the same.
me field of application of nonmagnetic steel has
been broadened in recent years, for example as structural
materials for constructing magnetic floating type high speed
railway (so-called a linear motor car), atomic reactors,
various electric component parts or the like. Suitable
nonmagnetic steel can be obtained by selecting its composition
to have austenitic structure. Typical example of such a
steel is austenitic stainless steel. In addition, Hadfield
steel (containing 0.9- 1.3% by weight of C and 11-14% by
weight of Mn) is a famous one. In the following description,
all percentages of the elements are % by weight based on the
total weight of ~he nonmagnetic steel. As improvements thereof,
such low carbon, high manganese nonmagnetic steels are known
as Mn-Cr steel (for example DIN x 40 Mn-Cr 18 steel), Mn-Cr-Ni
steel (for example DIN x 55 Mn Ni Cr 14 steel), and Mn-Cr-Ni-V
steel (for example DIN x 45 Mn ~i Cr V 1376 steel), etc.
The linear motor cars are prosperious in future
and such railway system requires a large quantity of nonmagne-
tic steel as guideway structures or reinforcing steels for
manufacturing railway beds so that ~ddition of such expensive
alloying elements as Ni and V is not advantageous. Such
nonmagnetic steel is also required to have low thermal
expansion coefficient and low electric resistivity in addition
to nonmagnetic property. Moreover it is also required that the
permeability should not increase even after cold working eg.
However, the prior art nonmagnetic steel can not satisfy these
requirements.
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7580
Accordingly, it is the principal object of this
invention to provide low cost nonmagnetic steel having a low
thermal expansion coefficient comparable with that or ferritic
steel or lower, a high yielding point and a low permeability
which would not increase after machining and method of manu-
facturing such nonmagnetic steel.
Another object of this invention is to provide a
novel method of manufacturing nonmagnetic steel at a low
cost having a low thermal expansion coefficient comparable
with that of ordinary steel (mean thermal expansion coefficient
of 1.0 - 1.3 x 10 S/C at a temperature of 0~ - lOO~C), a high
yielding point (0.2% proof stress) of higher than 36 Kg/mm2
and a permeability of less than l.l% after cold working.
Accordingly, the nonmagnetic steel is suitable for
use as guide structures and reinforcing steels of railroad beds
of the floating type high speed railway, structural members for
constructing fusion reactors, various electrical components,
etc.
According to one aspect of this invention there is
provided nonmagnetic steel having a low thermal expansion
coefficient, characterized by consisting of less than 0.5%
by weight of C, less than 2% by weight of Si 20 - 30% by
weight of Mn, and 0.005 - 0.04% by weight of N and the
balance of iron and impurities, wherein the following
relationships between the amounts of C and Mn are simultaneously
satisfied.
Mn (%) ~ 16 x C (%) + 18
Mn (%) >-12 x C (%) ~ 21.5
According to another aspect of this invention there
is provided a method of manufacturing nonmagnetic steel having
a low thermal expansion coefficient and a high yielding point,
characterized by comprising the steps of:
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11~758~
preparing slab or ingot containing 0.5% by weight of
carbon, less than 2% by weight of silicon, 20 - 30% by
weight of manganese, 0.005 - 0.04% by weight of nitrogen and
the balance o~ iron and impurities, in which the following
relationships are simultaneously satisfied
Mn (%) > 16 x C (%) + 18 .......... (1)
Mn (%)~ -12 x C (%) + 21.5 ........ (2)
heating said slab or ingot to a temperature of less
than 1220C;
hot rolling the heated slab or ingot; and
maintaining a finishing temperature to be less than
800C + 400CxC (%) depending upon the amount of carbon,
The nonmagnetic steel of this invention may further
contain less than 2% by weight of Cr.
In the accompanying drawings:
Fig. 1 is a graph showing a relationship between the
amounts of carbon and manganese of the present invention;
Fig. 2 is a graph showing a relationship between the
amounts of carbon and manganese necessary to obtain a stable
austenitic phase;
Fig. 3 is a graph showing the relationship between
the amount of manganese and the mechanical properties of high
manganese steels,
Fig. 4 is a graph showing the relationship between
the amount of manganese and the physical properties of high
manganese steels;
Fig. S is a graph showing an equithermal expansion
coefficient in a stable austenite phase;
Fig. 6 is a graph showing the relationship between
the tensile testing temperature and proof stress at a given
strain rate;
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114758~
Fig. 7 is a graph showing two examples of the rela-
tionship between the rolling finishing temperature and the
yielding strength (0~2% proof stress),
Fig. 8 is a graph showing the relationship between
the amount of carbon and the finishing rolling temperature for
obtaining 0.2% proof stress,
Fig. 9 is a graph showing the relationship between
the thermal expansion coefficient of high manganese steel and
the amount of nitrogen; and
Fig. 10 is a graph showing the relationship between
the thermal expansion coefficient and the amount of chromium
of a high manganese steel.
The reason for limiting the ranges of the elements
are as follows.
More particularly C is an important element for
stabilizing austenite and as the amount of C increases the
amount of another austenite stabilizing elements can be
reduced. Moreover, C is effective to increase the strength
of austenite steel. For example, the yielding strength
increases 1.8 Kg/mm2. Too much C, however, degrades hot
workability and/or requires to increase the amount of Mn for
the purpose of obtaining desired thermal expansion coefficient.
This is not only uneconomical but also impairs curring
machinability.
Mn is cheeper element than another austenite sta-
bilizing elements so that the austenite stability of high Mn
steel is mainly determined by a balance between the amounts
- of C and Mn. In other words, as the amount of C increases,
austenite can be stabilized with lesser amount of Mn. In high
carbon steel the lower limit of Mn is about 7% but it is
necessary to increase the amount of Mn to at least 20% in
order to maintain low thermal expansion coefficient as will
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, . . . . . . .. . . ...
1~4758~
be described later. Incorporation of Mn in excess of 30%
increases the cost of manufacturing and complicates the
manufacturing steps. For this reason the upper limit of Mn
was determined to be 30%. The result of regression analysis
regarding the thermal expansion coefficient of 30 types of
steel shows that C has a tendency of increasing thermal
expansion coefficient whereas Mn has a tendency of decreasing
the same. The ranges of C and Mn that result in a thermal
expansion coefficient comparable with that of ordinary steel,
that is less than 1.25 x 10 /C (average of from 0 to 100C)
are expressed by equation (1) and shown by the region above a
line a-a in Fig. 1.
As above described both C and Mn act as austenite
stabilizing elements and increase in the amounts of these
elements decreases permeability. The ranges of C and Mn
which can produce stable nonmagnetic steel after 20% cold
reduction were determined by degression analysis and are shown
as a region above a line b-b shown in Fig. 1, This relation-
ship is expressed by equation (2).
Thus, in order to have a thermal expansion coefficient
of less than 1.25 x 10 5/oC which is nearly equal to that of
ordinary steel and a permeability of less than 1.1 after cold
working, it is necessary to limit the amount of Mn as above
described and to simultaneously satisfy equations (1) and (2).
The balance relationship between the amounts of C and
Mn which are necessary to obtain stabilized austenite phase
after 20% cold working or 80% cold working is shown in Fig. 2
which shows that the balance relationship is nearly equal for
20% and 80% cold workings.
The Hadfield steel or its improved low carbon high
manganese steel, which are typical of the prior art nonmagnetic
steels have a thermal expansion coefficient of from 1.5 to
1 8 10-5/
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~47580
Less than 0.005% of N tends to lose the austenite
stability whereas more than 0.04% c N impairs the hot
workability of steel. For this reason, the range of N was
selected to be 0.005 to 0.04%.
While Ni, Cr and V are elements effective to increase
the strength of austenitic steel, from the standpoint of
economy, it is advantageous to select Ni to be less than 2%,
Cr to be less than 2% and V to be less than 0.5%. Incorporation
of these elementswhitin these ranges does not impair extremely
the thermal expansion coefficient, one o~ the features of
this invention.
Some examples of the nonmagnetic steel of this
invention will now be described as follows.
Table I below shows the mechanical and physical
properties of hot rolled steels embodying the invention and
comparative steels. Each sample were prepared from a 25 Kg
steel ingot which was then hot rolled.
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1~758~
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-- 7 --
11475130
Fig. 3 is a graph showing the relationship between
the amount of Mn and the elongation and the tensile strength
of steels respectively containing 0.02%, 0.25% and 0.54% of
carbon. m ick lines of the graph show stable austenitic phase.
As shown by curves shown in the lower portion of Fig. 3, the
tensile strength increases with the amount of carbon whereas
the austenite phase becomes more stable with the increase in
the amount of Mn and the tensile strength decreases.
Fig. 4 shows physical characteristics of the steels
containing indicated amounts of carbon. Thus, the mean thermal
expansion coefficient decreases with the amount of carbon,
but increases with the amount of Mn. The result of regression
analysis shows that, in a composition containing a stable
austenite phase there is the following relationship between
the thermal expansion coefficient a and the amounts of C and
Mn.
a = 1.80 + 0.48C - 0.03Mn (3)
Equithermal expansion coefficient calculated by
eq~ation (3) is shown in Fig. 5. ~umerals shown in Fig. 5
represent the mean thermal expansion coefficient xlO 5/oC
between O~C and 100C.
As shown by the middle portion of Fig. 4 the
resistivity is large and increases with the amounts of C and
Mn. Since, the resistivity is generally large in austenite
steels, such increase in resistivity does not cause any serious
problem.
As shown in the upper portion of Fig. 4 the permea-
bility becomes low regardless of the amounts of C and Mn so
long as the steel has a stable austenitic structure, which is an
advantageous property for nonmagnetic steel. Sample G shown
in Table I contains 1.7% of Cr. But this sample also has a
low thermal expansion coefficient of 0.98 x 10 5/C as well as
~1~7580
su*ficiently low resistivity and permeability that can
accomplish the object of this invention. Steels incorporated
with Ni or V were also investigated and it was found that steel
containing less than 2% of Ni or less than 0.5% of V also has
a low thermal expansion coefficient which can accomplish the
object of this invention.
To prepare the nonmagnetic steel of this invention
care should be taken for the soaking or reheating temperature
when hot rolling an ingot or bloom having a composition
described above. ~lus, Fig. 6 shows the relationship between
the tensile testing temperature and high temperature reduction
of area when a high Mn austenitic steel is heated and then
subjected to a high temperature tensile test. As can be noted
from Fig. 6, at temperatures above 1250C the reduction of
area decreases greatly which results in cracks at high
temperatures. In a large steel ingot since segregation of the
components is remarkable, it is advantageous to heat it at a
temperature below 1220C.
The rolling condition has a greatly influence upon
the yielding strength (0.2% proof stress) of high Mn austenitic
steel. More particularly when the austenitic steel is rolled
in a low temperature range the grain size of the product can
be greatly reduced.
Fig. 7 shows the relationship between the finishing
rolling temperature and the yielding strength (0.2% proof
stress). Thus it is possible to increase the yielding strength
by more thant 10 Kg/mm2 for controlling the finishing
temperature to be below 900C for 0.23C - 21.4 ~ steel and to
be less than 850C for 0.12C - 27.4 Mn steel.
We have made a number of experiments regarding the
amount of carbon and the finishing rolling temperature.
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~47581~
The result is shown in Fig. 8 from whlch it can be noted that
in order to obtain a yielding strength of larger than 36 Kg/mm ,
the strengthening action caused by carbon should be taken
into consideration.
Generally speaking, the finishing temperature should be
be controlled in a range of from 800 to 950C and the finish-
ing rolling temperature should be selected to satisfy the
following equation (4).
Finishing temp. FT (C)~ 800 + 400 X C (%) ..... (4)
Some preferred examples of the method of this
invention will now be describëd together with control examples.
25 Kg steel ingots each having a composition as shown in the
following Table II were rolled under rolling conditions also
shown in Table II.
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1147580
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1147580
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-- 12 --
1147580
As the steels of group I show, when the heating
temperature is elevated beyond 1220C, surface defects are
formed but as the groups I through L show, when the heating
temperature is lowered below 1220C no surface defect appears
on the surface which has been one of ~he problems in the method
of manufacturing high Mn steel plates. Lowering of the
finishing rolling temperature results in an excellent yielding
strength and in sample L3 a satisfactory yielding strength was
obtained meaning a great saving of expensive alloying elements.
The rolling conditions were selected such that the cumulative
reduction rate at a temperature below 1000C increases continuous-
ly as the finishing temperature is decreased. For eY~ample, the
rolling conditions were selected such that a 60% reduction can
be obtained at a finishing temperature of 750C. Regression
analysis showed that the mean thermal expansion coefficient.a
between 0 and 100C can be shown by the equation (3).
The equithermal expansion coefficient calculated
according to this equation has already been shown in Fig. 5.
The thermal expansion coefficient is not appreciably affected
20 by the amounts of Cr and N as shown in Figs. 9 and 10. me
thermal expansion coefficients of high N and high Mn steels
are mainly determined by the amounts of C and Mn thus proving
that application of equation (3) is possible.
As above described, the invention provided improved
nonmagnetic steel having a low thermal expansion coefficient
. comparable with or lower than that of ferritic steel and a
permeability which is sufficiently low in an as rolled state
and does not rise even after cold working. Moreover, it is
possible to obtain inexpensive nonmagnetic steel without the
necessity of incorporating a large amount of such expensive
alloying elements as Ni and V. Consequently, the nonmagnetic
steel of this invention is suitable for use as guideway
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~1~7S80
structures and reinforcing steels of railway beds of magneti-
cally Eloating type high speed railways, nuclear power plants
and various electric component parts.
Moreover according to the method of this invention,
it is possible to prevent surface defects which have been
inevitable in the manufacture of high Mn steel. me method
of this invention is applicable to manufacture thick plates,
shaped steel stocks or steel bars and rods.
.
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